Automatic radio direction finder



June 2, 1964 J. w. SCHWARTZ 3,135,958

AUTOMATIC RADIO DIRECTION FINDER Filed Sept. 26, 1961 3 Sheets-Sheet 1 June 2, 1964 J. w, SCHWARTZ 3,135,958

AUTOMATIC RADIO DIRECTION FINDER Filed Sept. 26, 1961 3 Sheets-Sheet 2 nvm :1v/v, 594550, sea {.frff VII-E: .7529.

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Ellb- United States Patent C 3,135,958 AUTOMATHC RADIO DREC'HON FENDER .lames W. Schwartz, Phoenix, Ariz., assigner, by mesne assignments, to Regency Electronics, Indianapolis, Ind. Filed Sept. 26, 1961, Ser. No. 140,872 6 Claims. (Ci. 3dS- 113) This invention relates to radio direction iinders and more specically to improvements in a radio direction finder wherein signals of a directional antenna and isotropic antenna in quadrature to one another are added to produce a signal having unambiguous position information therein.

Radio direction finders using directional antenna are well known to the art. The directional antenna can, for example, be a loop antenna which is rotated such that the amplitude of the signal received by the antenna will vary from a maximum to a minimum during each 180 of rotation. The direction of the transmitting station can be determined from the maximum or minimum, but only after the inherent ambiguity is resolved. To resolve the ambiguity, it has been the practice to add to the signal from the directional antenna a second signal from an antenna which is non-directional. For instance, if the directional antenna is of the type which respondsl to the magnetic component of an electromagnetic wave being transmitted, a non-directional antenna of the type which responds to the electric field component of the wave may be provided and the signals from both added. Since the electric and magnetic vectors are in time quadrature to one another, the phase of the electric vector received may be shifted 90 by passive elements so that it will either add to or subtract from the signal received by the directional antenna depending upon the position of the directional antenna relative to the transmittingr station. In that manner only one maximum and one minimum signal receiving position of the rotating directional antenna is simulated for its complete rotation through 360 to resolve the ambiguity.

The eifect of the addition of the signals from the two antennae is to cancel one of the minima present in the signal from the directional antenna so that the net signal has only a single minimum or null. The practice has been to detect the null and thereby determine the null signal receiving position of the directional antenna to obtain relative bearing to the transmitting station. A typical radio-direction-nding system is described in a United States Patent 2,232,096 which provides a means for detecting a single null and another means responsive to the null detecting means for tiring a thyratron which provides an electrical pulse that is employed to cause an appropriate visual representation of the position of the directional antenna relative to the transmitting station at that time, thereby providing a relative bearing to the transmitting station. The connection of the isotropic antenna in such systems will shift the null position of directional antenna. That is, the out of phase components measured by the ambiguity-resolving antenna will have components therein which are not at 90 with respect to the signal delivered by the directional antenna. Secondly, the precise position of the null point depends in part upon the relative magnitudes of the signals taken from the directional antenna and the non-directional antenna. Thus, a further null` position inaccuracy is placed in the system. Furthermore, since the prior systems are null-seeking systems, they must seek the absence of a signal. Accordingly, noise will interfere with the operation of the system.

The principle of the present invention is to base the measurement of the ambiguity-resolved signal on the phase of the envelope of the unambiguous signal. That is to say, where the directional antenna is rotated at 1500 rpm., for example, the output wave shape will have a 25 cycle modulation. This will be the fundamental component of the envelope wave shape. The phase relationship of this envelope changes proportionally with a change in direction between the measuring antenna and the transmitting station to be detected.

The phase of the 25 cycle component of this signal is independent of the relative proportion of sense antenna signals and directional antenna signals and also is independent of the out of quadrature components injected into the system by the sense or ambiguity-resolving antenna. Furthermore, since it is now a phase relationship that is to be determined, noise will have much less influence on the measurement than it would in the case of a null-seeking system since the system of the present invention integrates over such disturbances.

In one embodiment of the invention, the envelope of the fundamental component of the received signal is appropriately selected as in a low frequency band pass filter. The signal is then clipped to produce a square wave which is thereafter passed through a differentiating circuit to produce a series of pulses. The series of pulses then pass through a second clipping stage whereby the remaining pulses are those generated at a time related to the phase of the signal. As the phase of this fundamental shifts due to a shift in the relative position between the measuring antenna and the transmitting station, the'phase relationship of the pulses will shift proportionally.

These pulses can then be used tol trigger any appropriate type of visual display means which will, for example, display a light on a pelorus scale calibrated in terms of direction. For example, the pulse can trigger a neon tube which is scanned by a rotating disc having an aperture therein where the disc rotates at the effective speed of rotation of the directional antenna system. Thus a pulse of light will be synchronously presented through the aperture each time the aperture passes the position corresponding to the direction of the transmitting station to be detected.

In a second and preferred embodiment, a harmonic component at twice the fundamental frequency is utilized to derive a series of pulses. The fundamental frequency component is then employed to enable only those pulses of a selected polarity generated at a time related to the phase of the signal to trigger the display means.

Accordingly, a primary object of this invention is to provide a novel radio direction inding system having improved accuracy.

Another object of this invention is to provide a novel radio direction finding signal which does not require the use of null detecting techniques.

A further object of this invention is to provide a novel radio direction system which is substantially independent of noise.

A further object of this invention is to provide a novel radio direction finding system which is unaffected by the out of quadrature component in an ambiguity-resolving antenna signal.

A further and important feature of the invention is to provide a novel radio direction finder system which responds to the phase of the fundamental frequency of the envelope of a detected signal.

These and other objects of the invention will be apparent from the following description when taken in connection with the drawings in which:

FIGURE l is a schematic block diagram of a rst embodiment of the present invention.

FIGURE 2 shows a modification of the first embodiment of the invention wherein an actual rotating antenna with the result that there is a poorly defined null. This problem will be overcome by the present invention. Again, in FIGURE it is assumed that the voltage from the isotropic or ambiguity-resolving antenna is much less than the signal received by the directional antenna, with the result that two minima are now produced in each cycle. Thus, it is seen that the null seeking equipments of the prior art are dependent in part on the relative proportion of signal magnitudes from the two antenna systems.

In accordance with the present invention, the attempt to actually measure a null position is abandoned, it being recognized that the envelope of the antenna voltage will shift in phase as the direction of the transmitting station changes with respect to the equipment. Accordingly, the envelope of the received signal is operated upon to identify the unambiguous direction of the transmitting station. Thereafter, as shown in FIGURE l, the unambiguous signal from the phase shifter and mixer 25 is applied to a band pass filter 26 which is tuned to the frequency of the fundamental of the envelope of the unambiguous signal. If the motor 23 drives the antenna-rotation simulator 22 at 1500 revolutions per minute, for example, the narrow pass filter will be tuned to only receive the 25 cycle modulation introduced by the simulated rotating loop antenna.

The Wave shape of this resultant signal is shown in FIGURES 11a, llb and llc for the conditions of FIG- URES 8c, 9 and l0, respectively. From a comparison of the three signals for the three different conditions shown it may be clearly seen that the phase of each of the signals is identical to one another regardless of any shift in null position or any difficulty in sensing a poorly defined null position. The signal from filter 26 is thereafter taken to a clipper 27 such as an over driven amplifier with cutolf limiting. The output of clipper 27 is the square wave shown in FIGURE 12a which is applied to a diferentiator 28. The differentiator 28 delivers sharp pulses as illustrated in FIGURE 12b corresponding to the leading and trailing edges of the input square wave. By passing the output of diiferentiator 28 through a second clipper 29 all but selected positive (or negative) pulses are eliminated at the output of clipping stage 29, the remaining pulses corresponding to the direction of the transmitting station, as scanning through 360 is operated continuously.

This pulse is then applied to an indicating device such as a neon tube 30 which is positioned behind a disc 31 that is rotated by the motor 23 in synchronism with the modulation frequency of the directional antenna signal. The disc 31 has an aperture 32 therein which rotates with respect to a transparent pelorus scale 33. Therefore, when the rotation simulator 22 produces a signal simulating a signal from a directional antenna aligned with the transmitting station, the neon tube 30 will produce a flash observable through the aperture 32 on the pelorus scale 33. The ash will occur in the illustrated embodiment 25 times per second which is well within the retentivity of the eye whereby a steady dot of light is observed.

It should be noted that a continuous non-ambiguous signal is provided by the present invention, while in nullseeking devices it is the practice to permit the ambiguous directional signal to be continuously presented (since sharp nulls are existent in such signals), and to switch in the ambiguity-resolving antenna only when sense is to be determined. In the present invention reliance is not placed on the null position so that the sense antenna can be constantly connected in the system without affecting it.

'It should be further noted with reference to FIGURE l that a receiver 34 can be connected to the mixer 25 and that the receiver could, for example, be a superheterodyne system connected to a speaker 35 so that a transmitting station being tracked can be identified.

As noted hereinbefore, the antenna system including directional antennas 20 and 2l of FIGURE 1 could be replaced by and are the full equivalent of the actual rotating loop antenna. Thus, in FIGURE 2 a loop antenna 36 is illustrated which is directly rotated by a synchronous motor 37 which rotates, for example, at 25 revolutions per second and drives a scanning disc 38 which is similar to scanning disc 31 of FIGURE l. In FIGURE 2, the signal measured by an antenna 24 is phase shifted in a phase shifter 39 and the two signals are thereafter added and directly connected to the superheterodyne receiver 40 where the signals are amplified. From the output of the last IF stage in the superheterodyne receiver 40 the amplified signal can be connected to the 25 cycle band pass filter and clipper combined schematically and represented by block 41. The advantage of this arrangement is utilization of the RF and IF amplification stages in the receiver to provide preamplification which would otherwise be required in the clipper 27 except for very strong signals received from a nearby station. The output square Wave signal is then applied to the differentiator comprised of capacitor 42 and resistor 43 and the clipper, diode 28. Thus, a unidirectional pulse train is applied to the neon bulb 30 which cooperates with the scanning disc 38.

A preferred embodiment of the invention is best illustrated in FIGURE 3 wherein two frequency components of the received signal envelope are used with a received signal of the type shown in FIGURE 10. A signal envelope of that type is assured by mixing only one fourth of the signal amplitude from the isotropic antenna 24 with the signal from the antenna rotation simulator 22. A first band pass filter 52 passes the fundamental 25 cycle per second component of the modulated signal while the second band pass filter 50 passes the first harmonic frequency of the modulated signal. When too great a portion of the signal from the isotropic antenna is added in the signal there may be a fiickering of the indication and some dispersion of the envelope Wave shape. In accordance with the preferred embodiment of FIGURE 3, and assuming that a motor 23 causes a 25 cycle per second modulation of the directional signal, by adding a relatively small proportion of the isotropic signal, a 50 cycle harmonic frequency is produced as illustrated in FIGURE 10 at the output of phase shift mixer 25. This 50 cycle signal is passed by the sharply tuned band pass filter 50 and is applied to a limiter and differentiator 5l in much the same manner as in the system illustrated in FIGURE 1 whereby a 50 cycle pulse train is obtained.

A second channel is then provided which includes the band pass filter 52 tuned to the 25 cycle fundamental signal. This signal is applied to the biasing means of amplifier 53 which receives the output of limiter and differentiator means 51 whereby a pulse signal will be passed and amplified by amplifier 53 only during the positive half wave of the sinuosidal biasing voltage from the 25 cycle band pass filter 52. Accordingly, a strong positive pulse will be delivered from gated amplifier 53 at a frequency of 25 cycles to operate the indicating device. The system of FIGURE 3 will be operable even when weak signals are received and when only a small amount of the isotropic antenna signal is utilized.

In the foregoing, I have described my invention only in connection with preferred embodiments thereof. Many variations and modifications of the principles of my invention within the scope of the description herein are obvious. Accordingly, I prefer to be bound not by the specific disclosure herein, but only by the appended claims.

I claim:

l. A radio direction finder comprising directional antenna means for modulating a radio signal in accordance with the direction from which the radio signal comes and phase detecting means connected to said directional antenna means for measuring the phase of the envelope of said modulated signal, the indicator means connected to said phase detecting means; said indicator means being operableto indicate direction as determined by the phasel of said envelope of said radio signal; said phasedetecting t n means; said second bandfpass filter being connected to i meanszincludingra first low frequency Vband pass lter t for accepting only the fundamental frequency of saidenvelope and a second-band pass f-requencyfilterfor ac-V cepting only a harmonic of said fundamental frequency;

a pulse circuit means, said second band pass filter being Y connected to said pulse circuit means for generating voltage pulses` at phases relatedk to the phase, of said harmonicV of said fundamental frequency; a gated means; said second band pass filter being connected toA said gated means; said gated means being connected tol saidindicator means;

said gated meansA being connected to said first band pass filter and being abled only when said first bandpassfilterV said modulated signal with respect to a predetermined' value; said phase detecting means including a first band pass filter for accepting only the fundamental frequency' of said envelope andra second band pass frequency filter for accepting 'only a harmonic of said fundamental fre# quency; a pulse circuit means; said second band pass filter being connected to said pulse circuit means for `generats ing voltage pulses at phases related to the phase of said harmonic of said fundamental frequency; a gated means; said second band pass filter vbeing connected lto said gated means; an indicator means; said gated means beingY connected'to said indicator means; Vsaid gated means be-A ing connected to said first bandpass filter and being abled only when said first band pass filter has a predetermined 3. In a direction finder; directionalantenna means having a modulated output voltage containing anull valuecorresponding to the kdirection of a source of radio sig nals and measuring means for measuring the direction of said directional antenna corresponding to said null value; a first band pass filter for accepting only the fundamental frequency of said modulated output voltage and a second band pass frequency filter for accepting only a n harmonic of said fundamentalfrequency; ya pulse circuit means; said secondhand pass filter being connected to said pulse circuit means for generating voltage pulses at phases related to .the phase of said harmonic ofy saidV fundamental frequency; a gated means; said second band pass filter being connected to said gated means; an indicator means; said gatedrmeans beingconnected'to said indicator means; said gated Vmeans being connected to said first band pass filter and being abled only whensaid first bandpass filter has a predetermined output; said measuring means including said first Yband pass filter for passing said fundamentalfrequency component of said modulated output voltage and pulse generating means for generating a pulse havingV a fixed phase relation with respect to said fundamental frequency component; and an indicating means; said indicating means being connected to saidpulse generatingn means and indicating'the direction of said source of radio signals corresponding to the position of said pulse. Y s l 4. In a direction finder; directional antenna means having a modulated output voltageV containing'a null value corresponding to the direction of a source of radio signals Voltage pulses at phases relatedftojthe phase of said harand measuring means for measuring the directionV of said directionalantenna corresponding to Vsaid null value; a

first band pass filter for accepting only the fundamentalV frequency of said modulated output voltagerand a sec- 0nd band pass frequency filter for accepting "only a harmonic of said fundamental frequency; a pulsercircuit Vsaid pulse circuitvmeansfor generating voltage pulsesfat v phases related-tothe phase f said harmonic of saidfuny v damentalfre'qUenCy; a gated means; said second band pass 41j I lter being connected to saidgated means; anindicat'or means; vsaidk gated vmeansfbeing connected toV said indi.- cator means; said gated means beingconnected'to Ysaid first bandpassV filter and being abled only Whensaid first'V band pass filter has aspredetermined voutput;rsaid measurV ing means including rsaid first band pass filter forA passing thev fundamental frequency component of saidr'nodulated' outputV voltage` and pulse generating means for generati ing a pulse vhaving' aV fixed phase relation With respect to saidrfundamental component; and an indicatingv means;

saidv indicating me'anssbeingV connected to said pulse gen-'tv eratingmeans and'indicating' thedirectionrofl said *sourceA` of radio signals correspondingfto the position of-saidKV pulse; `said directional antenna-'means includingr'an iso-y tropic rantenna-for 4eliminating direction ambiguity.v

5. In a directionfinde'r; 'directional antennafmeans hav-f"Y r ing a modulated-output voltage containing anull value s corresponding to theV directionof a source ofkradiosig-vl nals and measuring means for measuring the direction 'of` .Y

said directional antenna corresponding to said null value;

said measuring means-including afbandjpass filter forV passing one component of said modulated Vfoutputfvoltage and pulse generating means for generating arpulse ha vi lingV a fixedphaserelation with respect 'toV said-one com-` ponent; land an indicating means; said indicating-means .Y

s being` connected Vtorsaidpulse generatingrmeans andiinldic'ating the direction-of said source ofradio signals (cor`A f rresponding'to the position ofV saidV pulse; saidfone com# ponent comprising'jthe4 first 'harmoniocomponent of said `modulated output'voltage; said pulse generating means f including a 'second' band 'pass filter for passing the fundamental component of said modulated Voutput voltage; Ysaid pulse generatingmeans Vincluding vgated amplifier means controlled by saidkfundamental component ofsaid rnoclu'j lated output voltage.l v j v ,Y Y t v 6. A radio direction finder comprising directional antenna means for modulating a radio signal in accordance with the direction from which the radiosignal comes;V Y, and phase detecting-rneansconnected to'saidrdirectionalv Y antenna means `for measuring the phase ofthe envelope of saidrmodulated signal, and indicator means connected to said'phase detecting meansglsaidtindicator meansrbeing' operable to indicate Vdirection 4as determined bythe phase of said envelope ofsaid 4radio VsignahsaidV phase detect-V f 'i ingfmeans including a first low/frequency band pass filter for'V accepting only the fundamental frequencyvofVv said envelope and arsecondV band passfrequency filter for Vac cepting only a harmonic of said fundamental frequency; a"

pulse circuitkmeans; saidv second bandrpass filter .beingy connected, to said pulse circuit means for Vgenerating monic of said fundamental'vfrequency; aV gated means; saidl second bandpassfil'ter being connected to said gatedY means; an indicator means; said gated means beingcon nected to said Vindicator means; said gatedmeans being i connected to said first band pass filter and being operable between oney of an abledf or disabledposition to the other Y Luck; Apr.v 1, V1947l 

1. A RADIO DIRECTION FINDER COMPRISING DIRECTIONAL ANTENNA MEANS FOR MODULATING A RADIO SIGNAL IN ACCORDANCE WITH THE DIRECTION FROM WHICH THE RADIO SIGNAL COMES AND PHASE DETECTING MEANS CONNECTED TO SAID DIRECTIONAL ANTENNA MEANS FOR MEASURING THE PHASE OF THE ENVELOPE OF SAID MODULATED SIGNAL, THE INDICATOR MEANS CONNECTED TO SAID PHASE DETECTING MEANS; SAID INDICATOR MEANS BEING OPERABLE TO INDICATE DIRECTION AS DETERMINED BY THE PHASE OF SAID ENVELOPE OF SAID RADIO SIGNAL; SAID PHASE DETECTING MEANS INCLUDING A FIRST LOW FREQUENCY BAND PASS FILTER FOR ACCEPTING ONLY THE FUNDAMENTAL FREQUENCY OF SAID ENVELOPE AND A SECOND BAND PASS FREQUENCY FILTER FOR ACCEPTING ONLY A HARMONIC OF SAID FUNDAMENTAL FREQUENCY; A PULSE CIRCUIT MEANS, SAID SECOND BAND PASS FILTER BEING CONNECTED TO SAID PULSE CIRCUIT MEANS FOR GENERATING VOLTAGE PULSES AT PHASES RELATED TO THE PHASE OF SAID HARMONIC OF SAID FUNDAMENTAL FREQUENCY; A GATED MEANS; SAID SECOND BAND PASS FILTER BEING CONNECTED TO SAID GATED MEANS; SAID GATED MEANS BEING CONNECTED TO SAID INDICATOR MEANS; SAID GATED MEANS BEING CONNECTED TO SAID FIRST BAND PASS FILTER AND BEING ABLED ONLY WHEN SAID FIRST BAND PASS FILTER HAS A PREDETERMINED OUTPUT. 